High efficiency modulator with automatic regulator of power source



D. G. LITTLE 2,064,019

HIGH EFFICIENCY MODULATOR WITH AUTOMATIC REGULATOR OF POWER SOURCE Dec, 15, 1936.

Filed May 3, 1933 WITNESSES: INVENTOR 5 flan/d6 L/2//?.

BY L.

'ATTO'RNEY l Faiented Dec. 15, 1936 UNITED STATES t mid FiCE HIGH EFFICIENCY MODULATOR WITH AUTOMATIC REGULATOR POWER SOURGE of Pennsylvania Application May 3, 1933, Serial No. 669,174

2 Claims.

My invention relates to radio transmission stations and particularly to means for supplying plate current to the vacuum tube devices in such stations.

It has become frequent practice to use for an amplifier a device which draws practically zero current in the absence of a signal and draws a substantial current when responding to one. Thus, in a radio sending station, the amplifier, when acting to modulate the output of the oscillator, draws current but at other times draws substantially none. Ordinarily, the increase in current drawn causes diminution in the potential of the source of supply of the plate current.

This produces an unfortunate result in the radiated signal because less power is available at the time of maximum modulation, which is the time when it is particularly important that full power be available.

It is an object of my invention to remedy the foregoing defect.

Stated in another way, it is an object of my invention to counteract the consequences of poor regulation in the supply system.

It is another object of my invention to obtain from the plate current of the modulator an effect which shall counteract the fall in potential of the source; of direct current supply. I accomplish this by causing this effect to increase the supply of power from the alternating current line to the power rectifier.

I have described the application of my invention to a radio-sending station but it can be usefully applied wherever an amplifier of the type drawing little current in the absence of a signal and substantial current when responding to a signal is used in a situation in which it is desirable to automatically regulate the voltage supplied to the plate circuit either of said amplifier or of any tubes associated therewith.

Public address systems, electrical carillons, and

radio receiving sets are examples of such situations.

Other objects of my invention and the details of the structure employed will be apparent from the following description and the accompanying drawing which is a diagram of the circuits and the apparatus employed. In the drawmg, the line I represents a source of alternating current which supplies the primary 2 of a transformer constituting part of the power rectifier. The power rectifier includes the secondary 3, rectifying tubes 5 and 5', connected 55 respectively to the two terminals of the secondary 3 and a connection from the midpoint of said secondary.

The output connections from the rectifiers 5 and 5' include a filter comprising an inductor 1 in series and condensers 8 and 9 across the output. If the tubes 5 and 5 are of small internal impedance, such as mercury vapor tubes, the condenser 8 may be omitted. This filter and the rectifiers may be considered together as the source of direct current analogous to the power pack often used with radio receivers.

The filaments for the rectifiers 5 and 5' are energized from a transformer 6 which is connected to the line I. A connection to the midpoint of the secondary of the transformer 6 constitutes one side of the filter and the conductor in extending from the filter constitutes the terminal of the direct-current source opposite the terminal represented by the conductor 4.

The connections from the line I to the primary 2 includes two coils l2 and I3 wound upon a common core I i in such sense that their magneto-motive forces produce additive effects in the core Hi. This core is equipped with a central leg carrying a coil 15 which is included in the conductor 4. Shunted around the coil I5 is an adjustable resistor It by which the average current through the coil l5 may be adjusted.

In the illustrated embodiment of my invention, the signal is represented by the sound delivered into the microphone 20. The potential changes corresponding to the signal are amplified by the tube 2|, and impressed upon the primary of a transformer 22. The secondary of the transformer 22 has a mid-tap connected to the negative terminal of a grid-biasing battery 23. The grids of the tubes 25 and 26 are connected, respectively, to the two terminals of the secondary of transformer 22, each half of the secondary being shunted by a high resistance 21 and 28, respectively. The battery 23 is of such a voltage that the normal potential of the grids of the tubes 25 and 26 is very near the cut-off point, whereby, in the absence of a signal, very little current flows in the plate circuits of these tubes.

The plates of the tubes 25 and 26 are connected to opposite terminals of the primary of a transformer 30, the midpoint of this primary being connected to the conductor It. The secondary of the transformer 39 delivers potential through a radio frequency choke 3! to a plate of an oscillator tube 32. The plate of tube 32 is also connected to the coil 33 of a tank circuit associated with the antenna 34. The grid of the oscillator 32 is connected through a condenser 35 to the end of the coil 33 opposite the plate connection.

An intermediate point of the coil 33 is connected through an adjustable condenser 36 to the filament of the tube 32, and also to ground as shown at 39. A condenser 40, in series with the condenser 35 and connected to the same terminal of the coil 33 as the grid, completes the tank circuit and the coupling thereof to the oscillator tube. A filament heating transformer 4i is shown for the tube 32. The primary of this transformer is connected directly across the line I. Obviously the filaments supplied by the transformer M and those supplied by transformer 6 could be supplied by a common transformer if desired. The transformer M, as shown, supplies the filaments of tubes 2I, 25 and 26 in addition to that of the tube 32, but it is obvious that separate transformers may be used if desired or whenever convenient.

A condenser 43 is illustrated as completing the plate circuit for tube 2|, one terminal of this condenser being connected to the midpoint of a resistor 44 across the terminals of the filament. It will be observed that the condenser 43 is in shunt to the source of direct-current power, since it is connected between the conductor I and, through the filament-heating circuit, to the conductor 4.

The connection of the conductor 4 to the two sides of the filament heating circuit, as shown, is by means of a resistor 45 connected across the filaments of tubes 25 and 26, but it is obvious that the resistor 44 or the connection to the midpoint of the secondary of the transformer 4! could be used with equal effectiveness if convenient. The resistor 45 also serves, in the illustrated system, to connect the positive pole of the battery 23 to the filaments of the tubes 25 and 26, but this connection also may be made in any desired way. It should also be noticed that the ground 39 grounds the filament of all the tubes 2i, 25, 26 and 32. The filaments of tubes and 5 are at positive directcurrent plate potential above ground. The midpoint of secondary 3 is at ground potential except for the drop across coil I5.

In the operation of the device, power is delivered from the line I to the primary 2. The potential at the primary 2 is less than that at the line i, because of the drop over coils I2 and I3. This drop will change with the inductance of these coils which will change with saturation of the core I4. The primary 2 delivers power to the secondary 3 which is rectified by the tubes 5 and 5 and delivered through the filter to the terminals 4 and II]. The filter removes the ripple which remains after the action of the rectifier. The potential delivered by the lead I8 is, therefore, substantially free from ripple.

At the point 48 the current supplied by the lead It! divides and a portion of it supplies the direct-current component in the plate circuit of oscillator 32. The average or direct current to tube 32 does not change between the condition of modulation or no modulation except due to distortion in tube 32 which is usually negligible. Another branch from the point 48 supplies the direct-current component in the plate circuit of the tube 2|. Changes in this current are small and need not be further dtscussed.

The remaining branch from the point 48 is to the middle of the primary of the transformer 30 and supplies current to the back-to-bacl: amplifier including tubes 25 and 26. This current, in the absence of a signal, will be small but, when the signal is present, the current through the tubes 25 and 26 will become of substantial amount, that is, while the signal persists, first one and then the other of these tubes draws a substantial current.

Modulation of the output of the tube 32 by the action of the voltages generated in the secondary of the transformer 30 is well understood and need not be described in detail here.

When a signal is present, the current through the tubes 25 and 26 increases, with the result that the current through the return conductor 4 increases, and, therefore, the magneto-motive force caused by the coil I5 increases. This causes an increase in the flux in both branches of the core I 4. This will bring the magnetization of the iron inside of coils I2 and I3 nearer the saturation point. During one-half cycle of the alternating current, the magneto-motive forces of coils I2 and I5 are additive and during the other half cycle they are opposed. The approach toward saturation during the additive relation is much greater than the departure from saturation during the other half cycle. On the average, therefore, an increase in current in coil I5 is accompanied by a decrease in the effective impedance of the coil I2. A similar effect takes place in coil I3 except that the approach toward saturation occurs during the opposite half cycle from that for the coil I2.

The consequence of a decrease in the apparent impedance of coils I2 and I3 is an increase in the voltage impressed across the primary 2 and consequenlty an increase in the voltage delivered by the rectifiers through the filter to the vacuum tubes.

Stated from a somewhat different viewpoint, when the apparent inductance of coils I2 and I3 is decreased the power factor of the connections from the line I to the source of directcurrent power is increased and, therefore, the power delivered from the line I is increased.

When a signal is being transmitted, therefore, the modulator uses more current, but instead of thereby causing a fall in potential at the terminals of the direct-current power source it causes a correction in the power factor of the connections between the line I and the rectifier, the potential delivered from the source of directcurrent power being thus kept at its normal value.

The direct-current potential impressed upon the oscillator 32 is thereby kept constant, with the result that the only changes in the magnitude of the radiations from the antenna 34 are those corresponding to the modulation. A greater faithfulness in the representation. of the signal is thus obtained.

It is difiicult to so design the device having core I4 that some alternating current potential is not induced in the winding I5. With an amplifier of the type shown, this causes no harm because the resulting currents in the two halves of the primary of transformer 30 offset each other and so introduce no hum into the secondary.

Many variations in detail will occur to those skilled in the art. The specific illustration and description of one embodiment of my invention is, therefore, not to be regarded as a limitation.

The only limitation intended are those required by the prior art or indicated in the accompanying claims.

I claim as my invention:

1. In combination, a pair of electron discharge devices having output circuits connected back-to-back, a load supplied from said output circuits, a source of voltage supply for the anodes of said electron discharge devices, said source being common to said output circuits and comprising a rectifier drawing power from alternatingcurrent lines a coupling between the input and output circuits of said rectifier for varying the input voltage to said rectifier in accordance with current in said rectifier output circuit.

2. In combination, a pair of electron discharge devices having output circuits connected backto-back, a load supplied from said-output circuits, a source of voltage supply for the anodes of said electron discharge devices, said source being common to said output circuits and comprising a rectifier drawing power from alternating-current lines means controlled by current in the output circuit of said rectfier for compensating for changes in input voltage to said rectifier produced by changes in current in said rectifier output circuit, said means comprising an inductor having a three-legged magnetic core with a winding connected in said alternating-current lines on two of said legs, and a winding connected in said rectifier output on the third leg. DONALD G. LITTLE. 

